Add _on_commit hook for tdb_cassandra.ThingBase.

r2/r2/lib/db/tdb_cassandra.py

@@ -629,6 +629,8 @@ class ThingBase(object):
 
         if not self._committed:
             self._on_create()
+        else:
+            self._on_commit()
 
         self._committed = True
 
@@ -744,6 +746,10 @@ class ThingBase(object):
            well"""
         pass
 
+    def _on_commit(self):
+        """Executed on _commit other than creation."""
+        pass
+
     @classmethod
     def _all(cls):
         # returns a query object yielding every single item in a